This invention relates to a receptor sheet for electrophotography. More specifically, it relates to a receptor sheet which is suitable for receiving a toner image transferred to the surface of an electrophotographic recording medium after formation thereon by a magnetic brush method using a carrierless one-component developer, said transferred image being subsequently fixed under pressure.
In recent years, a magnetic brush developing method using a one-component developer and a method of fixation under pressure have attracted attention in order to reduce power consumption in a copying machine, increase the maintainability and reliability of the copying machine, reduce the cost of the machine, and to impart quick starting properties. Since a toner containing a magnetic power is used in the magnetic brush developing method using a one-component developer, the transferability of the resulting image is poor when using a receptor sheet of the high-quality paper type used in conventional copying machines. It has the further defect that the copies obtained has a low image density and a disturbed edge. This phenomenon is especially pronounced in a high-humidity atmosphere, and poses a serious problem in practical application. This is because at the time of transfer, a transfer charge of a polarity opposite to the charge of the toner given to the sheet is injected into the toner beyond the interface between the sheet and the toner, for example, by corona discharge of the corotron-type, and thus the toner is retransferred to the surface of the photosensitive material or reciprocates between the sheet and the surface of the photosensitive material. In general, the speed of movement of a transfer charge in paper increases with moisture absorption. Specifically, the insulating property of the paper is reduced upon moisture absorption, and the speed of movement of the charge increases. Also, when the fixation under pressure is performed by means of press rollers, disturbance may be caused in the toner image on the sheet surface at the time of the entry of the sheet into the gap between the rollers because the electrostatic attracting force between the sheet and the toner is small or when the fixed image is rubbed with a finger or paper, it readily drops off. Thus, such a method is not satisfactory for practical application.
A method comprising coating paper with an aqueous emulsion of a hydrophobic resin would be an effective means for increasing the insulating property of paper, reducing its humidity dependence and to improve pressure fixability. Suitable commercially available resin emulsions for this method include emulsifier-free emulsions of acrylate ester of vinyl acetate resins from the standpoint of insulating property. Since these emulsions do not contain a hydrophilic surface-active agent as an emulsifier, they form a hydrophobic, insulating continuous coating when coated on paper, and thus the transferability of an image is improved. The pressure fixability can be markedly improved by selecting soft to medium hard resin emulsions. The use of a self-crosslinking type emulsion, or a combination of a reactive emulsion and a crosslinkable thermosetting resin is also found to be effective. Many of such emulsions are emulsions of acrylate ester-type resins which have been used previously for textile finishing.
However, these emulsions have poor miscibility with pigments, especially with organic pigments. Furthermore, the stability of the paint is low. Thus, flocculation or gum-up of the emulsion may occur, or in the case of on-machine size press coating, the drying cylinder and canvas may be contaminated. As stated hereinabove, the coating and impregnation of these emulsions on paper have not yet been fully achieved on a commercial basis, and the relatively high cost of the aforesaid emulsions also poses a problem.
In Pike et al. U.S. Pat. No. 2,487,060, there is disclosed a barrier coat for adhesive sheets or tapes which has an adhesive composition applied to a flexible porous backing such as paper, cloth, etc. The purpose of the barrier coat is to prevent excessive penetration of the adhesive composition into the backing which results in a waste of the adhesive material and reduces the quality of the final product. The barrier coat of Pike et al is made of a synthetic rubber, such as polychloroprene or butadiene-styrene copolymer, with fillers such as magnesium oxide and clay, as well as plasticizers, curing agents, and softening agents.
In Pelletier et al. U.S. Pat. No. 3,468,698, a pigment coated paper designed for offset printing is disclosed. An example of the coating composition of Pelletier et al is a mixture containing 8 to 10 percent styrenebutadiene latex admixed with 70 to 80 percent calcium carbonate and clay, together with smaller amounts of other materials. This coating composition was coated onto a paper at a rate of 61/2 to 9 pounds per 3,300 square feet, which is roughly equivalent to about 9.6 to 13.4 grams per square meter.
In Middletown et al. U.S. Pat. No. 3,655,608, there is disclosed a coated paper which is suitable for printing. The coating composition is a mixture of polymerizable binder material and a pigment pretreated with a hydroxyl- or hydrogen-reactive organic chemical. The binder of Middletown et al is an adhesive such as a polyester or polyether. Examples of the hydrogen and/or hydroxyl-reactive compounds are isocyanates and silanes. The binder component can be a monomer or a polymer, which can be crosslinked or cured after admixture with the treated pigment.